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I I I I I I I I I I I I I I I I I I I INTERPRETATION AND USE OF REPORT DATA Flood Frequency and Discharge The 10-, 50-, 100-, and 500-year flood events were used as the flood frequencies for this floodplain analysis. The data developed in this report, therefore, will be compatible for regulation purposes, H.B. 1041 designations and also for Federal Insurance Administration flood insurance rate studies. These various flood events have an average occurrence of once in the rwmber of years as indicated. For example, the 100-year flood occurs, on the average, once in a 100 year period. It has a one percent chance of being equaled or exceeded in any given year. The particular uses for the various flood events in addition to those stated above are as follows: lo-Year and 50-Year Flood Events- Information regarding these lower frequency floods is especially useful for future engineering studies and land use planning purposes relllted to minor road systems, minor channel improvements, the location of parks and recreational facilities, agriculatural lands and appurtenant structures. For structures and uses of this type on the smaller tributaries and in areas where the high risk of structural failure is economically feasible and the hazard to life and property nonexistent, the use of the lower frequency floods may be considered. 100-Year Flood Event- The 100-year flood event may also be used for engineering design purposes where a lower risk of failure than the 10- or 50-year flood is desired. However, the most important use of the 100-year flood event lies in floodplllin designation and land use regulation as set forth in the state statutes. The State of Colorado considers the 10o-year frequency flood as the flood event to be used in designing and protecting structures and -15- dwellings for human occupation. Therefore, all floodplain regulations are based upon the 100-year flood. Also, the area which would be inundated by the 100-year flood may be delineated as an area of state interest as set forth in the State's Land Use Bill, H.B. 1041. 500-Year Flood Event- The 500-year flood event is useful in making the public aware that floods larger than the 100-year flood can and do occur. The 500-year flood event can also be used for regulating high risk developments within the floodplain such as nuclear power plants, or the storage or manufacture of toxic or explosive materials. Tables 1A and lB list the discharges and water surface elevations for the 10-, 50-, 100-and 500-year flood events at selected reference points for White River and Sulphur Creek. Flooded Areas The 100-year frequency flood has been selected by the State of Colorado as the flood event to be used for floodplain delineation and regulation. Thus, the Flooded Area plates show only the boundary of the 100-year floodplain. At the request of Rio Blanco County officials, those areas within the 100-year floodplain which will be greater than 1.5 feet deep and where flood flow velocities will be greater than 3.0 feet per second have also been delineated as an aid in distinguishing between a high and a low hazard flood area. Since the base map for the Flooded Area plates is a 2' contour map the outlines of the other flood events, 10-, 50-, and 500-year, can be readily plotted using the information in Table 1. The area delineated on the Flooded Area plates as the 100-year floodplain meets the requirements of H.B. 1041 as an area of state interest. Upon official approval of this report by the Colorado Water Conservation Board, the area outlined by the 100-year flood boundary will be designated a floodplain area and may be regulated accordingly by the local officials.